Hi OTH,
I have built up a few single XML lights and have found light output starts to look good anywhere between 2 and 2.5 amps, after this the gain over drain of the battery doesn't seem worth it IMHO. The light heads don't get silly hot too quick & most cells can cope with this sort of drain without getting too stressed so as you thought 2.5A is about where I would go
BTW it's thanks to seeing your CuLite feature a few years ago that I now have a lathe, many led 'spares' lots of Ali rod/tube, many unfinished projects, many 'future' projects and a mildly unhealthy lumen habit! Cheers.

I have recently made a new light that is a triple XML run by a b3flex. In my experiments I found the sweet spot to be around 2000mah. That seemed to be the best compromise of power to heat output. It was also the best compromise in battery runtime at 4x18650 2500mah batts. Giving 1.5 hours. So I runt mine in tri-mode and mostly run on medium setting such is about 1000mah and then use the high 2000mah. I also run the medium cutter triple 35mm optic which gives a lovely wide coverage and has since enabled me to drop the bar light as I get great depth of field as apposed to a more spot like beam.

I'm running 2 xmls, neutral white, and find I need 2500mah for a good amount of light. If you are running cool white you'd probably be good at 2000. Running an 18v 4000mah Lipo I barely put a dent in the pack @ 2.5 hours runtime. Cells are generally a around 3.9 volts left after these rides. No issues with heat, except one ride where the temps was in the mid 60's, riding slow twisty technical stuff. That was the only ride where the light felt noticeably warm.

Thanks for the feedback guys, all good comments and certainly food for thought concerning balancing lumens vs runtime etc.

cagliari,

BTW it's thanks to seeing your CuLite feature a few years ago that I now have a lathe, many led 'spares' lots of Ali rod/tube, many unfinished projects, many 'future' projects and a mildly unhealthy lumen habit! Cheers.

Apologies for being responsible for bringing so much grief and misery into your life!

It depends on helmet vs. bar, how many LEDs and how much battery capacity you can carry. For a helmet light with a 2 or 3 cell battery, I wouldn't go above 2A because of runtime and cooling reasons. On the bar, where you can carry a beefier battery and a bigger housing, I run mine at 3A max, but use the L4 setting on the h6flex (~1.9A?) most of the time so it doesn't overpower my helmet light at 1.5A. Both with 2 XM-Ls.

But in terms of absolute efficiency, around 2A is pretty much the sweet spot. If you can carry the no. of cells required, then go for more XM-Ls at 2A than fewer at 3A.